Soy sauce is made by first producing koji, which is mixed with salted water to produce moromi. The moromi is fermented for up to a year or more, and the liquid is expressed from the fermented batch to product the liquid soy sauce. This process is well known and has been practiced for centuries. A critical step in the process is that of producing the koji. Koji is conventionally produced by inoculating a combination of soy beans and cereal grains with a koji culture, and allowing that culture to grow on the koji mixture for time sufficient to provide a substantial koji culture growth.
The koji culture can vary in its contents of specific organisms, and indeed, the culture may be taken from a previous koji production. Irrespective of the particular koji culture, the culture is a mold, and its growth rate is sensitive both to temperature and the moisture content of the koji mixture (the substrate). Thus, if the temperature of the substrate is below about 15.degree. C., or above about 42.degree. C., or if the moisture content of the substrate is below about 13%, the mold will essentially cease to grow. Further, oxygen is consumed in the mold growth, and it is critical that adequate oxygen levels always be maintained, or otherwise the mold growth will be considerably slowed.
It is important to maximize mold growth in the shortest possible time since it is this mold which produces the enzyme that is ultimately responsible for producing the soy sauce flavor. Accordingly, in the traditional process of producing koji, the koji mixture of soy beans and cereal grain is inoculated with the koji culture, and the inoculated mixture is plced on open trays with a bed depth usually not exceeding two inches. Often, the trays are also perforated trays in order to increase the airflow around the inoculated koji mixture. During mold growth, the inoculated koji mixture is agitated, e.g. stirred by hand, both to provide additional oxygen and to lower the temperature of the inoculated mixture. As the mold grows, heat is generated and in substantial bed depths internal temperatures can exceed 45.degree. C. and stop all mold growth. This is, of course, a laborious and time-consuming process.
The mold growth is optimized when the koji mixture (the substrate) has a moisture content between about 20% and 40%, optimally just under about 30%, e.g. 27 to 29%. On the other hand, at moisture contents of about 42% or greater, the water activity of the substrate is sufficient to produce bacterial growth, which will sour the koji mixture and spoil that mixture for producing soy sauce. Thus, there is a practical upper limit on the amount of moisture which can be contained in the koji mixture. This moisture limit, however, results in other practical problems. Thus, as the inoculated koji mixture remains on open trays and is stirred or otherwise agitated during the mold growth, the moisture content of the koji mixture continues to decrease and may easily reach moisture contents out of the optimum growth range. Also, in extreme cases, the moisture content may drop so low that mold growth essentially ceases.
As can therefore be appreciated, the traditional process for making koji is really a compromise between competing factors. The bed depth of koji mixture must be kept shallow in order to avoid heat build-up during mold growth and to supply oxygen for mold growth. However, the shallow bed depth and the frequent stirring required increases the rate in which moisture is removed from the koji mixture. In addition, this compromise requires considerable space and labor.
The art has sought to overcome these difficulties in producing koji, and many suggestions in this regard have been made. Thus, the use of bactericides have been suggested in order to avoid bacterial growth and spoilage at higher moisture contents. The use of citric acid and tartaric acid have been suggested in this regard. Also modifying the koji substrate with ammonium acetate and lower alcohols have been suggested. Similarly, it has been suggested to use a calcium salt or a magneseum salt in combination with steaming of the substrate. Sulphite salts and sulfurous acid have also been suggested, even including antibiotics.
However, all of these additives change the flavor of the resulting soy sauce, and the flavor is not that of the true traditional soy sauce without those additives.
In order to avoid flavor degeneration, it has been suggested that the soy beans can be steamed in a very precise manner prior to inoculation, whereby moisture contents of 30 to 70% are achieved. It is said that the critical steaming, as opposed to the usual pressure cooking and/or steaming, causes some denaturization of the protein, but here again, the difficulties with souring of the koji are not mitigated (see U.S. Pat. No. 3,647,484).
In another approach, U.S. Pat. No. 4,048,340 suggests a fairly complex "closed" koji system comprising a koji chamber for carrying circulating gas, into which steam is injected. A series of cyclones remove dust and other impurities from the circulating gas blown through the chamber by continuously running fans. A bypass in parallel with the chamber allows the circulating gas to bypass the chamber in response to temperature sensing devices located externally of the koji making chamber near the entrance and exit ports of the chamber. Thus, the amount of circulating gas which passes through the chamber is adjusted according to the desired temperature and humidity. However, as pointed out in that patent, closed systems are very susceptible to accumulating contaminating bacteria, with resulting spoilage of the koji.
As is well known whole soy beans, which contain oil, protein, carbohydrates and fiber are not efficient for producing soy sauce and have disadvantages. Essentially only the protein and carbohydrate are used in the mold growth, and especially the protein. The remaining fiber and oil not only constitute unneeded bulk which must be handled, but create an increased disposal problem. Further, the soybean oil is a wanted product which can not be recovered from the spent beans, and in some cases the contaminating oil can become rancid and given an unacceptable off-taste.
For these reasons, the art has long recognized that the defatted beans (in the form of dry soy "grits") would be an advantageous substitute for whole soy beans in producing koji. Thus, for any improved process for making koji to be substantially commercially advantageous over the conventional process, the improved process must be capable of using both whole soy beans and soy grits. This necessity of using both whole soy beans and soy grits is due to varying supplies of both and the need to interchangeable use either one or the other without substantial changes in the koji making process.
The desirability of using soy grits has compounded the problems, discussed above, in connection with whole soy beans. When soy grits are substituted for soy beans, the higher bulk density of the soy grits substantially decreases the natural air flow through the open trays and the soy grits can easily exceed the maximum temperature of 42.degree. C. during mold growth. Further, oxygen levels with the bed of soy grits can decrease to where the rate of mold growth is is substantially decreased. Thus, when soy grits are used in the conventional method of producing koji, the soy grits must either be stirred more often, which compounds the problems discussed above, or the bed depth must be substantially decreased, which not only decrease the production of the koji but increases the problem of rapid moisture removal and consequential decrease in mold growth. Accordingly, the use of soy grits has not been as commercially attractive as desired, and the unability to effectively handle both whole soy beans and soy grits has made prior proposals for improved koji production unsatisfactory.
It will therefore be appreciated that it would be of considerable advantage to the art to provide a method for producing koji which does not suffer from the above-noted disadvantages, particularly in regard to spoilage by bacterial fermentation, time-consuming and laborious stirring, the requirement for substantial space for the shallow bed trays, and the inability to use both whole soy beans and soy grits.